In a number of applications of practical importance, it is necessary to be able to focus rays emanating from an X-ray source into a small spot size. For so-called hard or relatively high-energy X-rays, this is a particularly difficult and challenging task. Various techniques for fabricating optical elements capable of focusing hard X-rays are described in a paper entitled "Microfocusing Optics for Hard X-Rays Fabricated by X-Ray Lithography" by A. A. Krasnoperova et al, SPIE Proc., vol. 2516, pages 15-26, 1995.
In particular, a number of structures made by integrated-circuit fabrication processes and suitable for focusing hard X-rays have been priorly proposed. Among these are a variety of periodic structures including Fresnel zone plates.
Heretofore, Fresnel zone plates for hard X-rays have been formed in a layer whose thickness is specified by a particular phase-shift requirement. In practice, the thickness of such a layer from which phase-type zone plates are fabricated has typically been at least about several micrometers (.mu.m).
To achieve an extremely small or high-resolution spot size with a Fresnel zone plate onto which X-rays of a specified energy impinge, the smallest feature of the zone plate structure must have a width that is comparable in size to the desired spot size. Thus, for example, if the features of the zone plate are formed in a three-.mu.m-thick layer and an aspect ratio (thickness-to-width dimensions) of about ten is manufacturable, the smallest feature that can be defined in the zone plate structure is approximately 0.3 .mu.m [or about 300 nanometers (nm)]. In that case, the zone plate is capable of focusing the specified hard X-rays to a spot size having a diameter of the order of 300 nm. But, as the energy of the incident X-rays increases, i.e. for harder X-rays, the required thickness of the zone plate increases in order to ensure the correct phase shift between adjacent zones. Given the same manufacturable aspect ratio, this results in thicker zone plates with larger feature sizes and hence larger spot sizes.
Accordingly, efforts have continued by workers skilled in the art directed at trying to devise improved structures suitable for focusing hard X-rays to a small spot size. It was recognized that such efforts, if successful, would provide an important instrumentality for use in various practical applications such as in non-destructively examining small features in integrated circuits.